I. Technical Field
The present disclosure relates generally to camera imaging systems and more specifically to devices and techniques for capturing images in a rolling shutter, stereo image acquisition system that may be included on a vehicle.
II. Background Information
Camera based driver assistance systems for use in vehicles may include monocular object detection systems that rely primarily on a single camera to collect images. Because the images in these types of systems are captured from a single point of view, direct determination of distance to a target object can be challenging. Therefore, monocular object detection systems may rely upon estimation techniques to indirectly determine distances to a target object based on information about the object class and/or contextual information relative to the context of the object (e.g., aspects of a road plane on which the target object resides). In some cases, monocular systems may use pattern recognition to detect a specific object class prior to monocular range estimation.
Camera based driver assistance systems for use in vehicles may also include stereo systems that employ two cameras. In some systems, these cameras may be mounted side-by-side where epipolar lines are aligned with the horizontal image scan lines. Such a system may use a dense disparity map to create a 3D map of the environment. The system may then use this 3D representation for foreground and/or background segmentation, for instance, to find candidate regions for further processing. The system may also use the 3D representation to locate interesting objects or to estimate range and/or range-rate to detected objects. Such stereo systems may work well with close and medium range targets and in good weather, and may give depth maps on general targets. However, such stereo systems may experience difficulties during adverse weather conditions or where cluttered scenes exist. Additionally, these systems may have difficulty imaging objects at longer distances from the vehicle.
Some imaging systems, such as systems described in U.S. Pat. No. 7,786,898, may fuse information from both a monocular system and a stereo system. This type of system may include a primary camera responsible for target detection/selection and range estimation. A secondary camera may provide stereo-range on selected targets for purposes of target verification.
Some stereo systems may include an asymmetric configuration that may combine stereo-depth and monocular depth together. For instance, two asymmetric cameras (e.g., with different fields of view (FOV) and focal lengths) may be employed for independent applications. Additionally, image information from these cameras may be combined to provide stereo depth. For cameras with global shutters, such stereo processing may involve, among other things, cropping the wider FOV camera, smoothing and subsampling of images, and/or rectification in order to provide a matching image pair.
Recent generations of image sensors, including those that may be used in automotive sensors, may include a rolling shutter. Such a rolling shutter may introduce complications in stereo image processing, especially in asymmetric stereo applications that use cameras having different fields of view. For instance, if both a wide FOV camera and a narrow FOV camera are aimed at a common scene, then the narrow FOV camera may overlap with only a portion of the FOV of the wide FOV camera. If both cameras acquire images as a similar number of image scan lines acquired at a similar line scan rate, then the acquired image scan lines in the area of the overlap in the fields of view of the two cameras will lack synchronization. Such a lack of synchronization may introduce difficulties in determining a correspondence of image points in a first image from the wide FOV camera with image points in a second image from the narrow FOV camera, which can lead to significant inaccuracies object distance measurements.